1. Technical Field
The present invention relates to a method of chemical decontamination for carbon steel member of a nuclear power plant and more particularly to a method of chemical decontamination for carbon steel member of a nuclear power plant suitable for application to carbon steel member of a boiling water nuclear power plant.
2. Background Art
For example, the boiling water nuclear power plant (hereinafter referred to as BWR plant) includes a reactor having a core disposed in a reactor pressure vessel (referred to as RPV). Reactor water (cooling water) supplied to the core by a recirculation pump (or an internal pump) is heated by heat generated due to nuclear fission of a nuclear fuel material in a fuel assembly loaded in the core and is partially turned to steam. The steam is introduced from the RPV to a turbine to rotate the turbine. The steam discharged from the turbine is condensed by a condenser to water. The water is supplied to the RPV as feed water. Metallic impurities are mainly removed from the feed water by a demineralizer installed in a water feed pipe so as to suppress generation of a radioactive corrosion product in the RPV. The reactor water is cooling water existing in the RPV.
Further, a corrosion product which is a base of the radioactive corrosion product is generated on a surface of a structure member of a BWR plant such as an RPV and primary loop recirculation system piping (referred to as recirculation system pipe), the surface coming into contact with the reactor water, so that stainless steel and a nickel based alloy of less corrosion are used for the main primary-system structure members. Further, overlay welding of stainless steel exists on an inner surface of the RPV made of low alloy steel, thus the low alloy steel is prevented from direct contact with the reactor water. Furthermore, part of the reactor water is cleaned up by a demineralizer of a reactor water clean-up system, thus metallic impurities slightly existing in the reactor water is removed positively.
However, even if such a corrosion countermeasure as mentioned above is taken, very little metallic impurities unavoidably exist in the reactor water, so some metallic impurities, as a metallic oxide, are adhered to the surface of each fuel rod included in a fuel assembly. The impurities (for example, a metallic element) deposited on the surface of each fuel rod cause a nuclear reaction by irradiation of neutrons discharged by nuclear fission of the nuclear fuel in each fuel rod and become radioactive nuclides such as cobalt 60, cobalt 58, chromium 51, and manganese 54.
These radioactive nuclides are mostly kept to be adhered to the surface of each fuel rod in a form of an oxide. However, some radioactive nuclides are eluted as ions into the reactor water depending of the solubility of the taken-in oxide and are re-discharged into the reactor water as an insoluble called a crud. The radioactive material included in the reactor water is removed by the reactor water clean-up system communicated with the RPV. The radioactive material not removed by the reactor water clean-up system is accumulated on the surface of the structure member (for example, pipe) of the nuclear power plant which comes into contact with the reactor water while circulating in the re-circulation system together with the reactor water. As a result, a radiation is discharged from the surface of the structure member, causing radiation exposure to an operator during the periodic inspection operation.
The exposure dose of the operator is controlled so as not to exceed the regulated value for each operator. The regulated value has been reduced in recent years and there is the need to decrease the exposure dose for each operator as much as possible.
Therefore, when the exposure dose during the periodic inspection operation is expected to be high, the chemical decontamination for dissolving and removing the radioactive nuclide deposited on the pipe is executed. For example, Japanese Patent Laid-open No. 2000-105295 proposes a chemical decontamination method of executing reduction decontamination using an aqueous solution (a reduction decontaminating solution) including an oxalic acid and hydrazine, decomposition of the oxalic acid and hydrazine, and oxidation decontamination using an aqueous solution (an oxidation decontaminating solution) including a potassium permanganate. The chemical decontamination method is executed for the pipe and the like of the nuclear power plant.
Japanese Patent Laid-open No. 2001-74887 describes a chemical decontamination method executed to a recirculation system pipe made of stainless steel which is connected to the RPV and a purification system pipe made of carbon steel member of the reactor water clean-up system which is connected to the recirculation system pipe. In the chemical decontamination method, a potassium permanganate aqueous solution is supplied into the recirculation pipe and the purification system pipe to execute the oxidation decontamination for the inner surfaces of those pipes. Thereafter, an aqueous solution including the oxalic acid and hydrazine is supplied to the recirculation system pipe and the purification system pipe to execute the reduction decontamination. After the reduction decontamination, the oxalic acid and hydrazine included in the aqueous solution are decomposed.
Further, Japanese Patent Laid-open No. 2004-286471 and Japanese Patent Laid-open No. 2004-170278 describe a chemical decontamination method of storing the decontamination objects such as the equipment made of stainless steel and pipe which are removed from the nuclear power plant in a decontamination bath and executing the chemical decontamination. In the chemical decontamination method, a mixed aqueous solution including a formic acid of a concentration ratio of 0.9 and an oxalic acid of a concentration ratio of 0.1 is supplied into the decontamination bath to decontaminate the decontamination objects and the reduction decontamination of the decontamination objects is executed in the decontamination bath by using the mixed aqueous solution. After completion of the reduction decontamination, hydrogen peroxide (or ozone) is supplied into the mixed solution and the formic acid and oxalic acid included in the mixed aqueous solution are decomposed by the hydrogen peroxide (or ozone).
Japanese Patent Laid-open No. 2002-333498 describes a chemical decontamination method. In the chemical decontamination method, the chemical decontamination, concretely, reduction decontamination of carbon steel member using an aqueous solution (a reduction decontamination aqueous solution) including an organic acid (for example, the formic acid) and hydrogen peroxide is executed. Furthermore, in the chemical decontamination method described in Japanese Patent Laid-open No. 2003-90897, the reduction decontamination for the carbon steel member is executed using the oxalic acid aqueous solution, and after the reduction decontamination, an acid aqueous solution (for example, a formic acid aqueous solution) is brought into contact with the carbon steel member. Therefore, at the time of the reduction decontamination using the oxalic acid aqueous solution, the ferrous oxalate generated on the surface of the carbon steel member is removed by action of the acid aqueous solution.
Japanese Patent Laid-open No. 62-250189 describes a chemical decontamination method of executing the reduction decontamination for equipment made of stainless steel of a primary cooling system device by using a solution including a malonic acid, the oxalic acid, and hydrazine.